[OpenLayers-Users] Conversion to Spherical Mercator - 100m error

[George.Muammar]

Hi Christopher thanks for your reply.

None of my data is is EPSG:4326 and neither is the map. The data is
EPSG:3004 and everthing else is spherical mercator. EPSG:4326 is only used
in the displayProjection. Is this ok ?

Yes, I have tested both Google and VE.

Google and VE actually correspond when I look at the roads layer. I had not
noticed this previously.
The Google imagery is bad (in this area) therefore I ignore it.

However my data is off. 
Note that I have imported this same data into Google *EARTH* (i.e. in
EPSG:4326) and it overlayed fine. I also overlay it on my orthophotos so I
am sure about it's accuracy.

I think it may be due to this problem with Proj.4. I am using the version
that comes with Mapserver binaries for windows (version number is not
specified). Judging from the dates of the binaries it may be 4.4.7. Is that
old ?

I tried googling the problem you mentioned but got no hits. Could you send
me some link or tell me how to go about diagnosing that problem in my setup
?

Thanks again.




Christopher Schmidt-4 wrote:
> 
> On Fri, Aug 08, 2008 at 01:02:31PM -0700, George.Muammar wrote:
>> The road layer (Teleatlas / Navteq) comes in UTM - WGS84 and I also
>> combine
>> it with my cadastral data. I play with these projections every day and
>> are
>> very confident with them  - but not so with the so called "spherical
>> mercator"
> 
> Using Google Maps in EPSG:4326 *must* lead to offsets within OpenLayers.
> There is no possible way to avoid this, as Google Maps data is
> *projected* data, and EPSG:4326 is a 'flat' projection, using lat/lon as
> planar units. The end result is that there is no way that you can
> correctly overlay data on top of Google Maps, Virtual Earth, or any
> other commercial baselayer.
> 
> However, reading the following, I realize you are using spherical
> mercator? At least, I Think? In which case, the answer is "yes, I have
> used this with 'very accurate maps', and had no problems getting pixel
> perfect overlays on top of Google Maps. However, I have also seen
> significant offsets, *especially* in VirtualEarth (in the 100M range)
> even in areas where I would typicaly expect better results.
> (Specifically, New Orleans in VE at some zoom levels is *bad*.)
> 
> I don't see anything wrong with your configuration here; Have you tried
> laying over Google instead?
> 
> Another comment is that it is possible that your proj.4 version may
> affect things: I've heard people reporting problems with older proj.4
> libraries not respecting the +nadgrids=@null hack.
> 
> Just a few thoughts.
> 
>> 
>> Does anybody have any opinion in the matter ? has anyone tried to overlay
>> Google / VE / Yahoo with very accurate maps ?
>> 
>> For the record I am doing the following: (maybe something wrong)
>> 
>> projections in Proj.4 epsg file:
>> 
>> # Spherical Mercator for Google etc. 
>> <900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
>> +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs
>> #	GAUSS BOAGA EST 
>> <3004> +proj=tmerc +lat_0=0 +lon_0=15.0 +k=0.999600 +x_0=2520000 +y_0=0
>> +ellps=intl +pm=0 +units=m  no_defs <>
>> 
>> 
>> I define my map thus:
>> 	var smproj = new OpenLayers.Projection("EPSG:900913");
>> 
>> 	mapOptions = {
>> 	    projection: smproj,
>> 	    units: "m",
>> 	    displayProjection: geoproj
>> 	};
>> 
>> 
>> my layers thus:
>>            var hyblayer = new OpenLayers.Layer.VirtualEarth( ....
>> 'sphericalMercator': true});
>> 
>> 		var pmwms = new OpenLayers.Layer.WMS( .....
>>                           {'projection': smproj});
>> 
>> 
>> 
>> -- 
>> View this message in context:
>> http://www.nabble.com/Conversion-to-Spherical-Mercator---100m-error-tp18898188p18898188.html
>> Sent from the OpenLayers Users mailing list archive at Nabble.com.
>> 
>> _______________________________________________
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>> http://openlayers.org/mailman/listinfo/users
> 
> -- 
> Christopher Schmidt
> MetaCarta
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> 
> 

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